Striker Maynard suffered a dead leg during Saturday's 1-1 draw with Coventry and is rated as doubtful to face the Hornets.
Danny Haynes and David Clarkson will be vying to replace the former Crewe man if he misses out, with Clarkson staking his claim by coming off the bench to equalise against the Sky Blues.
Campbell-Ryce, a recent signing from Barnsley, injured his calf during the warm-up on Saturday and is expected to miss out, but Cole Skuse could return after recovering from a stomach muscle strain.
Robins boss Gary Johnson could recall midfielder Ivan Sproule as his side look to improve on a poor run of just one win in their last 10 games.
Source: PA
Source: PA